The present invention is directed to scanning probe microscopes, and more particularly, to a method and apparatus for adaptively tracking one or more features(s) of a sample during successive SPM scans.
Scanning probe microscopes (“SPM's”), such as the atomic force microscope (“AFM”), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding image of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employs a probe device including a probe 12 having a cantilever 15 and tip mounted or formed on or near the free end of the cantilever. A scanner 24 generates relative motion between the probe 12 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (X, Y, Z). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
Scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) such as a piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
In a common configuration, probe 17 is coupled to an oscillating actuator or drive 16 that is used to drive probe 12 to oscillate at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 15. Probe 17 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 12 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. The actuator 16 may be coupled to the scanner 24 and probe 12 but may be formed integrally with the cantilever 15 of probe 12 as part of a self-actuated cantilever/probe.
The probe 12 may be oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 12, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 12, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al, U.S. Pat. No RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam generated by the sensing light source can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are processed by a signal processing block 28 (e.g., to determine the RMS deflection of probe 12). The interaction signal (e.g., deflection) is then transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 12. In general, controller 20 determines an error at Block 30, then generates control signals (e.g., using a P1 gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 12. The control signals are typically amplified by a high voltage amplifier 34 prior to, for example, driving scanner 24. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used. Controller 20 is also referred to generally as feedback where the control effort is to maintain a constant target value defined by setpoint.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers located on-board or off-board the AFM, that receives the collected data directly or indirectly from the controller and that manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation involves moving the sample and/or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. As discussed briefly above, scanning typically occurs in an “X-Y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “Z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence justifying the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and/or used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one practical mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe, or harmonic thereof. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e., the force resulting from tip/sample interaction, typically by controlling tip-sample separation (a controlled distance between the probe and sample). Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs and other SPMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs and other SPMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
A method of operating an SPM by identifying a feature of the sample from the sample surface data and automatically performing a zoom-in scan of the feature based on the identifying step is described in U.S. Pat. No. 7,868,966 to Su et al., hereby incorporated by reference in its entirety. As described in U.S. Pat. No. 7,868,966, the method operates to quickly identify and confirm the location of a feature contained in a region of interest so as to facilitate performing a directed high resolution image of the feature.
Current techniques for tracking features in successive scans typically rely on tracking fixed data patterns. That is, they assume that the feature contained in the region of interest will remain essentially unchanged, with respect to shape, size, orientation, and other attributes of the feature in each successive scan. Some systems attempt to compensate for thermal drift of actuators that cause small apparent movement or distortion of the feature from scan-to-scan. These systems, however, are incapable of “adaptively” tracking a feature if that feature displaces over time by changing feature attribute(s) within the scanned area. Accordingly, movement of such features on the sample, movement of the sample and/or drift of the actuator by amounts as little as ˜0.10 nm per minute can lead to a failure to track the feature entirely over a series of scans. This failure can lead to severe image distortion and even failure to locate the feature in a given scan.